88 Dr. Thomson on Arsenic. [Feb, 



This is obviously equivalent to 1 atom of acid = 14-5 + 5 

 atoms oxide = 50 + 32 atoms water = 36. 



We see from the preceding statement that the composition of 

 these salts is not inconsistent with the notion that the weight of 

 an atom of arsenic acid is 14'5 ; but a new analysis of them 

 would be desirable. Want of the requisite specimens prevents 

 me from having it in my power to undertake it, 



IV. Arseniate of Iron. 



There occur in the copper mines of Cornwall small cubic 

 crystals of a dark-green colour, which Mr. Chenevix recognised 

 to be an arseniate of iron usually contaminated with a portion of 

 copper. This salt (abstracting the impurities) he found com- 

 posed as follows : 



Arsenic acid 14-50 



Protoxide of iron 20-91 



Water 4-83 



Now an integrant particle of protoxide of iron weighs 4-5. I 

 concei\e, therefore, that the salt in question is a compound of 

 1 atom arsenic acid = 14-5 + 5 atoms of protoxide of iron 

 = 22-5 + 4 atoms water = 4-5. This salt then is not incon- 

 sistent with the notion that an atom of arsenic acid weighs 14-5. 



V. Arseniate of Lead. 



Beautiful specimens of this salt occur in Huel Unity mine, 

 near Redruth, in Cornwall. It has a wax-yellow colour, and is 

 crystallized in large six-sided prisms. INIr. Gregor, who analyzed 

 these ci-ystals, found their constituents (abstracting a small 

 quantity of muriate of lead) as follows : 



Arsenic acid 14'5 



Protoxide of lead 35-7 



Now this is equivalent to 1 atom acid = 14-5 + 2-i atoms 

 protoxide of lead = 35 (for an atom of protoxide of lead weighs 

 14). The composition of this salt then is not inconsistent with 

 the notion that an atom of arsenic acid weighs 14-5. 



There is an arseniate of lead which precipitates in a white 

 powder, when arseniate of soda is dropped into nitrate of lead. 

 This powder is a compound of 



Arsenic acid 7-25 



Protoxide of lead 14-00 



But it is not inconsistent with the notion that an atom of 

 arsenic acid weighs 14-5. We have only to consider the salt as 

 a subbinarseniate or a compound of 1 atom acid + 2 atoms oxide 

 of lead. 



Thus I have gone over all the crystallized nrseniates v.ith 

 which I am acquainted, except the arseiiiate of ammonia, without 



